Field of Invention
The present invention relates to an electric machine.
Related Art
The electric machine is the most commonly used driving device. According to the functions, the electric machine can be divided into the motor and generator. In particular, the motor can convert the electric or magnetic power into mechanical power, and it has been widely applied to the machines of various fields.
In general, the motor includes a rotor and a stator, and the rotor can be driven to rotate by the magnetic flux between the stator and rotor so as to generate the required mechanical energy. In order to provide the path for magnetic flux, the conventional motor is configured with a plurality of stator teeth, and the coil for cooperating with the inner rotor is wound in the stator slots disposed between the stator teeth. However, this kind of electric machine has some drawbacks. For example, most coils can not be wound inside the stator slots in advance, so that the manufacturing speed can not be improved and the production cost can not be reduced. Moreover, the conventional design of the stator slots provides a gap between the teeth for winding coils during the manufacturing process. In other words, a stator slot opening is configured between the stator teeth. These gaps will cause the cogging torque as the rotor rotates, which results in the unsmooth rotation of the rotor.
In more details, the cogging torque is mainly caused by the variation of the magnetic attraction between the rotor and the stator teeth, and is related to the stator slot openings. When the coil disposed on the stator slot is applied with current, each stator tooth will generate a corresponding magnetic pole, thereby generating the expected magnetic attraction. The maximum magnetic attraction can be generated as the rotor's magnetic flux surface is facing directly toward the stator tooth. Afterwards, when the rotor starts to rotate and the magnetic lines pass through the stator slot opening (that is, the rotor rotates from the position facing directly toward the stator tooth to the position slanting to the stator tooth), the rotation of the rotor will be dragged by the magnetic attraction change, which is called the cogging torque. The cogging torque may cause some drawbacks such as the unsmooth operation of the motor, back EMF, noise (due to teeth vibration), lower torque output, and the likes. Moreover, in the generator application, the cogging torque will cause a resistance as the generator is starting or rotating, thereby affecting the efficiency of the generator. Therefore, it is desired to minimize the cogging torque and its effect in the electric machine.
As mentioned above, the conventional improvement method for minimizing the cogging torque is to modify the stator structure of the electric machine. For example, using the axial motor and the stator teeth are fabricated by casting, and then assembled so as to minimize the gaps between the stator teeth. However, although this approach can reduce the effect of the cogging torque, it is still not commonly used due to the high cost of the casting procedure. Another approach is to use the radial motor and place a staggered winding over a plurality of stator teeth. However, this approach requires a complex coil winding, which is complicated in manufacturing. Besides, the length of the coil is longer and multiple teeth must be driven at the same time, which requires higher driving voltage. Furthermore, the electric machine made by this approach still has a certain cogging torque. Alternatively, it is possible to design a slotless electric machine by using complex windings only to form the stator. Although this design does not need the casting procedure for fabricating the stator, the complex and expensive staggered coil windings are still needed.
Therefore, it is an important subject to provide an electric machine that can wind the coil in advance and has minimized cogging torque, simple structure, low cost and higher efficiency.